Help me please evaluate a stress under bolt head

[Mr_Curious]

Hello everyone.

I am designing a machine which has a bolt connection, and there is stress on one of the part under the bolt head, exceeding a yield strength.
The stress is caused by bolt head preload pressure only, because external loads have no influence on the point of interest.
The yield strength is 300 MPa and the max stress on the part is 310 MPa.

Many books say that it is not allowed having a stress above yield strength, whereas I haven't seen any mention about considering a stress under a bolt head.
Searching the internet I've seen that in reality the stress under a bolt head might be much higher due to surface deviation between a head and an opposite surface, also the surface roughness, in which the support does not occur on the entire contact surface but on the tops.

In my case, I can not to use a bigger bolts due to the lace of space and weight limits.
I assume that it is no problem with this place in the future.

I need some advices and thoughts about that please.

Details:
Bolt- M20
Preload force is- 118kN.
Bolt surface area is- 0,000386m2
The result is- 305 MPa.

FEA analysis result is attached:

 

[3DDave]

In cases like non-uniform contact the material yields until the pressure is below the yield stress. Also, it is typical to chamfer the edge of bolt holes to avoid contact with the fillet radius under the bolt head.

 

[desertfox]

Hi Mr Curious

Before I say anymore can you explain why the bolt surface area is in m^3 because it should be in m^2 which might make a difference to the result you are seeing.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Mr_Curious]

Hi Desertfox.

It was mistake. m2 is right.

 

[Ninja_Newyork]

HI Mr.curious,

Why dont you claculate the newber strain on that particular location to find out whether this were real stress or not.

Normally as a rule of thumb, the calculate neuber starin would be less than 1%, then we can consider this as a local stress.

Regards,
Anand,G

 

[desertfox]

Hi Mr Curious

Well from checking your calculation, ie bolt area under the head and the bolt preload it appears to me that that is the average stress compression on the part so yes something needs to change, now two options off the top of my head are place a washer under the bolt head to increase surface area or reduce down the preload on the bolt, given that you have ruled out larger bolts but what about using smaller bolts and increase the bolt number. So in my book what you appear to have is an average stress under the bolt head and not just a local high stress due to surface deviations, therefore the joint design needs to be addressed, I don’t follow your comment that the external forces have no influence at the point of interest can you please explain that better, if indeed the external load doesn’t have an influence at the point of interest then you could reduce the bolt preload.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[MJCronin]

Curious ...

I agree with desertfox .... Excessive local stress underneath bolt heads is the primary reason we use washers ...

This is an ancient problem that has been addressed in the past ... Ancient Aliens taught mankind about the advantages of washers in 3600 BCE

Why haven't different types of washers been explored ?

Why is there not a chamfer at the top of the bolt hole ?

https://www.boltscience.com/pages/why-use-washers.pdf

MJCronin
Sr. Process Engineer

 

[MintJulep]

Pretty picture. Why not add it to the post so people can see it, rather than force them to download it?

Without any information about how things were modeled it is impossible for anyone to provide useful commentary about the usefulness of the pretty picture.

Regardless, localized stress above yield --> material yields. Until the stress is below yield. Then it doesn't.

Tighen the fastener. Wait a few minutes. Hit things with a hammer. Tighten the fastener again.

 

[LittleInch]

Make the hole smaller?

How did you work out bolt head area?

I got 0.000486 as the maximum possible

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Mr_Curious]

To Desertfox

I mean that it does not influence on the area under the bolt head, but the bolt preload is needed for non-opening of the joint of the whole connection.(see a picture attached)

To MJCronin

I did not use ane washer bacause it increase self-loosening. And as for flanged headed bolts I can not use it because I don not have ehough place for them in the model.

To MintJulep

I did not manage to do upload any pictures directly to the window. The Image botton is not active in my browser. Sorry for inconveniences.

To LittleInch
I have measured the area in the M20 bolt I use.

 

[SWComposites]

The peak stress in the FEM is very likely a fictious stress that is an artifact of the model - course mesh, inaccurate geometry, contact surface, etc. ignore it. Besides, the mesh along the length of the hole looks like rubbish.

 

[desertfox]

Hi Mr Curious

Okay thanks I understand now but if the preload cannot be reduced then you either need more bolts or smaller bolts in an increased number,so that you can reduce the bearing stress under the bolt head while retaining joint integrity. I would personally try to add flat washers under the bolt head which may well solve the problem and is the cheapest solution in this instance, I have never heard of flat washers causing self loosening but I have heard of self loosening with regard to spring washers. Moving onto the area under the bolt head, I’m not sure I follow what you are measuring so I calculated the surface area in contact with the bolt head, this I did by taking the across flats of the M20 hexagon bolt head as a diameter ie 30mm in this case, I then took the bolt clearance hole which I assumed was 22mm diameter and proceeded to subtract the cross sectional area of a 22mm circle from the cross sectional area of a 30mm circle which gave me a bolt bearing surface of 326.768mm^2. Now if I divide the area into the given preload I end up with a stress of 361Mpa.
If there is not enough room to fit washers then you need to add more bolts as mentioned above because if the clamped material is yielding then the bolt will never reach the bolt preload you are asking for and if my calculation above is correct the stress over yield is worse than you predicted. Depending on the material being clamped you joint might work if the material being clamped work hardens but I wouldn’t rely on that.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Tmoose]

"I did not use any washer because it increase self-loosening."

That statement is often heard as kind of a rule of thumb. I think that is due to focused theoretical concerns about the added "faying surface" and the not-so-flat faces of low end commercial washers adding a little more "settlement" and even "embedment" to the detriment of installation preload.

I think a lovely flat, hard washer enhances any bolted joint's performance via luxuriously improved bolt torquing consistency and reduced relaxation, settlement an embedment.
And Here I believe the OP is, concerned specifically about embedment.

If a representative clamped part or even just a hunk of the clamped part's material is handy, drilling a couple of holes and tightening a few bolts would clarify the situation immensely. the FEA model needs verification anyway, Before or after.

I think this Image started appearing on the walls of US machine shops and assembly areas during or slightly after WWII.

https://pics.me.me/oh-no-you-dont-not-without-a-washer-nut-screw-52450581.png

 

[rb1957]

I agree ... it is bad practice not to use a washer. Particularly with the small fastener head you seem to have.

We're talking about localised yielding from a linear FEM ... usually an artifice of the FEM ... try smaller mesh size ... see the even higher stress !!

Have you considered beveling the hole ... removing the highly stressed material ?? But this would want a larger fastener head (or a washer).

If this is the part, what is happening in the fastener ? How are you loading the FEM ?

A material with fty = 300 MPa and ftu = 310 MPa sounds "sketchy" to me.

another day in paradise, or is paradise one day closer ?